1. Field of the Invention
The present invention relates to a corner waveguide-integrated electric circuit case, a waveguide/strip line converter, a circuit serving both as an oscillator and a mixer (hereafter referred to as an oscillator mixer), a circuit serving both as a multiplier and a mixer (a multiplier mixer), and an arrangement of a frequency-modulated continuous-wave (hereafter, FM-CW) radar. In particular, this invention is concerned with a corner waveguide-integrated electric circuit case and a waveguide/strip line converter, which are preferred as structures for connecting between antennas and a sensor unit in an FM-CW radar, an oscillator mixer and a multiplier mixer, which are preferred as circuits for a sensor unit in an FM-CW radar, and a new arrangement of an FM-CW radar.
2. Description of the Related Art
An FM-CW radar transmits an electric wave that is frequency-modulated by a triangular wave to an object via a transmission antenna, receives an electric wave reflected from the object via a reception antenna, generates a beat signal by mixing the received wave with the transmission wave in a mixer, and then analyzes the frequency of the beat signal in order to measure a relative distance and a relative velocity with respect to the object simultaneously. The FM-CW radar is mounted on the outer surface of an automobile, helicopter, or other vehicle because of its purpose of use. The FM-CW radar is, therefore, requested to be compact in terms of appearance and ease of installation. A millimeter wave band is usually used as the output frequency. Therefore, if a large number of circuit components is required, many expensive millimeter wave devices are employed, and an increase in cost ensues.
In an on-vehicle FM-CW radar, both a transmission antenna and a reception antenna are composed of a waveguide in which holes through which electromagnetic waves leak out or invade are bored at specified intervals, and a parabolic reflector lying behind the waveguide. The transmission and reception waveguides are stood up in a V-shaped form so that wave fronts from the waveguides cross. A sensor unit containing the circuits of a transmitter and a receiver is placed below the antennas. The whole radar section including the transmission and reception antennas and the sensor unit is shaped like a letter Y.
The radar is mounted on a front grille. The size, or especially, height of the radar must be as small as possible in terms of limited installation space and appearance.
As an attempt to cope with this demand, a bent waveguide or a corner waveguide is used. Specifically, a waveguide linked with the sensor unit is positioned horizontally in order to reduce the height of a radar.
This structure requires a bent waveguide or a corner waveguide, which poses the first problem that the number of parts increases and the height can be reduced only on a limited basis.
As described previously, each of the transmission and reception antennas is connected to a sensor unit via a hollow waveguide. The circuitry in the sensor unit is constituted by a microstrip lines made up of a dielectric board, a grounded conductor formed on the bottom of the dielectric board, and strip lines formed on the top of the dielectric board. At a junction between the transmission or reception antenna and the sensor unit, therefore, a waveguide/strip line converter must be provided to convert the transmission mode between the hollow waveguide and the strip line.
The waveguide/strip line converter usually has such a structure that a strip line board, which is realized with a dielectric board having a grounded conductor on part of its back and strip lines on its surface, projects into a hollow determined by the inner wall of a hollow waveguide from a side opening in a stationary fashion. The strip line board is secured by attaching the entire grounded conductor to the inner wall of the side opening of the hollow using conductive adhesive or solder.
When an equipment in which the waveguide/strip line converter having the aforementioned structure is incorporated is manufactured according to a mass production system, some products may be provided with excessive conductive adhesive or solder which will ooze out duly. In addition, although a strip line board must be fixed at a precise position, the above structure may cause a variation in the length of a projecting portion among products.
The shape of the line pattern in the projecting portion of a waveguide/strip line converter is designed to provide a maximum conversion characteristic (VSWR) for a product that has no oozing of conductive adhesive and is aligned precisely. If conductive adhesive oozes out or a fixing position deviates, the VSWR (voltage standing wave ratio) deteriorates. This results in the second problem that a desired conversion characteristic is unavailable.
In a basic configuration of an FM-CW radar, a sensor unit consists of a voltage-controlled oscillator, a directional coupler, and a mixer. The voltage-controlled oscillator inputs a modulating signal of a triangular wave, and outputs a millimeter wave signal that is frequency-modulated by a triangular wave. The millimeter wave signal is transmitted to a transmission antenna. Part of the millimeter wave signal is branched by the directional coupler, and mixed with a received millimeter wave signal coming from a reception antenna by the mixer. A baseband signal whose frequency corresponds to a difference in frequency between the transmitted wave and received wave is then fetched. A device used for a millimeter wave oscillator is, generally, expensive. Alternatively, the voltage-controlled oscillator generates a frequency-modulated signal whose frequency is a divisor of the transmission frequency, and then a multiplier multiplies the frequency-modulated signal.
The voltage-controlled oscillator, mixer, and multiplier have been constructed independently using different active elements. This poses the third problem that a sensor unit must be realized as a large circuitry made up of numerous expensive millimeter wave devices.
When two systems of FM-CW radars each having the aforesaid configuration are mounted on a vehicle, one of the FM-CW radars uses transmission and reception antennas, which are installed on the front frame of the vehicle, to measure a relative distance and a relative speed with respect to a vehicle running ahead, while the other FM-CW radar uses a transmission antenna placed to face the oblique forward ground and a reception antenna which is installed so as to receive part of electric waves reflected irregularly from the ground, to perform almost the same processing as one FM-CW radar. This permits measurement of a speed of a vehicle as well as a relative speed. If the speed of a vehicle can be measured, an absolute speed of a vehicle running ahead can be calculated and displayed, and, in conjunction with a measured value of a rotating speed of a wheel, a slip occurring on a tire can be detected and the degree of the slip can be measured. Thus, the range of use of an FM-CW radar further expands.
An FM-CW radar having the aforesaid configuration has a characteristic that the output signal of a voltage-controlled oscillator is changed not only in frequency but also in amplitude relative to a variation in amplitude of a control signal. Therefore, an amplitude-modulated (hereafter, AM) component whose frequency is the same as the frequency of a modulating signal (FM-AM conversion noise) is superimposed on the output signal. This frequency is very close to the frequency of a signal resulting from FM detection of a reflected signal. As a result, the signal-to-noise ratio deteriorates. A switching radar has been proposed as a solution to the above problem.
The switching radar comprises a rectangular wave generator that generates a rectangular wave whose frequency is sufficiently lower than the frequency of a carrier and at least twice as large as a frequency corresponding to a sum or difference between beat frequencies generated by a mixer, and a switch that is driven with the rectangular wave and outputs a frequency-modulated wave as a signal whose amplitude is modulated with an on/off signal. When the rectangular wave signal is mixed with an output of the mixer, the FM-AM conversion noise occurring in the voltage-controlled oscillator is eliminated effectively. Consequently, a signal with a high signal-to-noise ratio results.
As described previously, two systems of electric circuits are needed to enable measurement of a speed of a vehicle. This contradicts a demand for downsizing and price cutting. As for the switching radar, it does not effectively utilize power when switched off.